1. Field of the Invention
The present invention relates to a piezoelectric ceramics to be applicable widely in the field of a resonator, a pressure sensor, or the like.
2. Description of the Related Art
A piezoelectric element is a material having the piezoelectric effect of changing the electric polarization by the external stress, and the inverse piezoelectric effect of generating strain by the electric field application. The piezoelectric element is adopted in a sensor, a resonator, an actuator, or the like, for measuring the pressure or the deformation.
Most of piezoelectric materials practically used now are commonly ferroelectric elements having a perovskite structure based on PZT (PbZrO3xe2x80x94PbTiO3 solid solution) of a tetragonal system or a rhombohedral system, or based on PT (PbTiO3) of a tetragonal system. By adding various sub components thereto, various characteristics are provided as requested.
However, most of the PZT-based or PT-based piezoelectric materials have a 300 to 350xc2x0 C. Curie temperature in a practical composition. In contrast, since a process temperature in a present soldering step is, in general, 230 to 250xc2x0 C., a piezoelectric material with a 300 to 350xc2x0 C. Curie temperature can easily generate characteristic deterioration in the soldering step. Besides, in the case where a solder not containing a lead (lead-free solder) is used practically, the process temperature in the soldering step is further higher. Therefore, it is extremely important to have the Curie temperature of the piezoelectric material higher.
Moreover, since the lead-based piezoelectric materials contain a large content (about 60 to 70 wt %) of a lead oxide (PbO) having an extremely high volatility even at a low temperature, they are not preferable from the viewpoints of ecology and pollution prevention. Specifically, in producing the lead-based piezoelectric materials as ceramics or a single crystal, a heat treatment such as firing and melting is inevitable, and thus the lead oxide as a volatile component can be volatilized and dispersed by an extremely large amount in an industrial level. Furthermore, although the lead oxide discharged in the production step can be collected, most of the lead oxide contained in the piezoelectric materials dispatched to the market as industrial merchandises cannot be collected in the present situation. Therefore, in the case where it is discharged widely in the environment, it should inevitably be the cause of pollution.
As a lead-free piezoelectric material, for example, BaTiO3 of a perovskite structure belonging to a tetragonal system is well known. However, since it has a low Curie temperature of 120xc2x0 C., it is not practical. Moreover, JP-A-9-100156 discloses a (1xe2x88x92x) (Bixc2xdNaxc2xd)TiO3xe2x80x94xNaNbO3 solid solution of a perovskite structure; however, those having a Curie temperature higher than 370xc2x0 C. are not included.
As a piezoelectric element capable of having a Curie temperature of 500xc2x0 C. or more, for example, a bismuth layer compound (a compound shaped in bismuth layer) is known. Since the bismuth layer compound has a high Curie temperature, once it is polarized, it is thermally stable and thus characteristics sufficient for a sensor for high temperature can be obtained. On the other hand, a problem is involved in that sufficient piezoelectric characteristics can be hardly obtained for difficulty of the polarization itself. Furthermore, a lead-free bismuth layer compound involves a problem in that Qmax important in the case where it is adopted in a resonator is small. Qmax is tanxcex8max wherein xcex8max represents the maximum value of the phase angle. That is, it is the maximum value of Q (=|X|/R) between the resonant frequency and the antiresonant frequency wherein X denotes the reactance and R denotes the resistance. With larger Qmax, the vibration can be stabilized, and further, the vibration is enabled with a low voltage.
A report on improvement of Qmax of a lead-free bismuth layer compound is disclosed on pages 97-98 of the lecture abstracts of the 16th Meeting on Ferroelectric Materials and Their Applications (May 26-29, 1999). In this abstract, (Sr1xe2x88x92xMex)Bi4Ti4O15, wherein Me represents Ba, Ca, La, Sm, or Gd, with Ba and Ca added in the range of xxe2x89xa60.1, Sm and Gd Xxe2x89xa60.4, and La xxe2x89xa60.5, is provided as the lead-free bismuth layer compound. In the abstract, Qmax is measured in the thickness extensional fundamental vibration mode. FIG. 2 show the improvement of Qmax by addition of an La, and decline of Qmax by addition of Ba or Ca.
In the case of adopting piezoelectric ceramics in a resonator or a filter, small temperature dependency of the resonant frequency is required. However, piezoelectric ceramics having Qmax sufficiently large for the practical use as a resonator and small temperature dependency of the resonant frequency has not been reported so far.
An object of the present invention is to provide lead-free piezoelectric ceramics having a high Curie temperature, the excellent piezoelectric characteristics, and small temperature dependency of the resonant frequency.
The above-mentioned object can be achieved by the invention of the below-mentioned items (1) to (5).
(1) Piezoelectric ceramics comprising bismuth layer compounds containing MII, Bi, Ti, Ln and O, wherein MII represents at least one element selected from the group consisting of Sr, Ba and Ca, and Ln represents at least one element selected from the group consisting of lanthanoids. The piezoelectric ceramics contain MIIBi4Ti4O15 typed crystals wherein a mole ratio of Ln/(MII+Ln) is 0 less than Ln/(MII+Ln) less than 0.5 and a mole ratio of 4Bi/Ti is 4.000 less than 4Bi/Tixe2x89xa64.030.
(2) The piezoelectric ceramic according to the above-mentioned item (1), containing Mn oxide.
(3) The piezoelectric ceramic according to the above-mentioned item (2), wherein a content of Mn oxide is less than 0.62 wt % in terms of MnO.
(4) The piezoelectric ceramic according to any of the above-mentioned items (1) to (3), further containing Co oxide.
(5) The piezoelectric ceramic according to the above-mentioned item (4), wherein a content of Co oxide is less than 0.7 wt % in terms of CoO.
Further, the above-mentioned object can be achieved by the invention of the following items (6) to (8).
(6) Piezoelectric ceramics comprising bismuth layer compounds containing MII, Bi, Ti, Ln and O, wherein MII represents at least one element selected from the group consisting of Sr, Ba and Ca, and Ln represents at least one element selected from the group consisting of lanthanoids. The piezoelectric ceramics contain MIIBi4Ti4O15 typed crystals wherein a mole ratio of Ln/(Ln+MII) is 0 less than Ln/(Ln+MII) less than 0.5, and the piezoelectric ceramics further contain Y oxide.
(7) The piezoelectric ceramics according to the above-mentioned item (6), wherein a content of Y oxide is contained by 0.5 wt % or less in terms of the Y2O3.
(8) The piezoelectric ceramics according to the above-mentioned item (6) or (7), containing Mn oxide and/or Co oxide.